The present invention relates to a color optical printer head which can optically print characters or patterns on photosensitive materials and more particularly to an optical shutter device which is suitable for colored hard-copy devices.
Liquid crystal elements are currently in wide use for digital display in electric calculators or wristwatches and application of liquid crystal elements for dot-matrix display which can arbitrarily display arabic numerals, Kanji-characters, Kana-characters or picture images is now being studied. Many of those elements are for black-and-white display using TN (torsional nematic) type. FIGS. 1(a) and (b) show the principle of the TN method. As shown in the figure, a nematic liquid crystal is interposed between two glass substrates 1, 2 and the molecules 5 of the liquid crystal have a long axis which is parallel to the substrate face but is distorted in orientation increasingly along the direction of thickness (at the time the electric field is not impressed, FIG. 1(a)). The torsion angle from one glass substrate 1 to the other substrate 2 is 90.degree.. In front of and behind such liquid crystal cells are arranged polarizing plates 3, 4 respectively which cross each other perpendicularly in polarization direction. When white light enters such liquid crystal panels from behind, the light passes through a polarizing plate 3 to be polarized in one direction. When the light enters the liquid crystal, it propagates along said liquid crystal molecules 5 to be rotated in polarizing direction by 90.degree.. The polarizing direction therefore coincides with the polarizing direction of the polarizing plate 4 which is positioned ahead and the light proceeds without being blocked to pass through the polarizing plate 4. When an electric field is impressed on said liquid crystal in the direction of thickness, the liquid crystal molecules 5 are aligned with the direction of electric field in the long axis thereof as shown in FIG. 1(b). If white light is made to enter said element from behind, the light passes through the plate 3 to be polarized in one direction and then propagated through the liquid crystal as it is. The direction of the polarized light is kept perpendicular to the polarizing direction of the plate 4 and hence blocked by the plate 4. The liquid crystal of the TN method is utilized as display element by using such mechanism that the liquid crystal cell becomes opaque when an electric field is impressed while it becomes transparent when an electric field is not impressed. As most of the liquid crystal elements of the TN method, however, are used for black-and-white display by means of opaque-and-transparent elements, the demand for color display has been keenly felt as the use of liquid crystal elements is remarkably expanded. Color display will improve attraction, recognition and diversity of the display and further provides a wider field of vision for some methods.
There have been proposed color display methods by means of liquid crystals using various principles. Typical are the ECB method (electrically controlled birefringence) using the birefringence property of liquid crystals, the GH method (guest-host) which adds a bi-color element to the liquid crystal, and the color TN method in which the elements of the aforementioned TN method are covered with a birefringence color filter or a bi-color filter. FIG. 2 shows the liquid crystal elements of the color TN method wherein multicolor display becomes possible by combining color filters of red, green and blue in the element. More particularly, nematic liquid crystal 12 is inserted between two parallel glass substrates 10, 11 and transparent electrodes 13, 14 in the form of a strip are vapor-deposited in a plural number on the inner faces of said glass substrates 10, 11 in a manner so that they cross each other perpendicularly. When electric voltage is applied to combinations of opposing faces of the transparent electrodes 13, 14, arbitary patterns of matrix become displayable. Color filters of R, G, and B are formed on transparent electrodes 14 shown in the lower part in the figure or the transparent electrodes which are closer to the surface of the liquid crystal display element. The color filters are of color addition mixture and the color filter R transmits red light, the color filter G green light and the color filter B blue light. The reason why color filters R, G and B are provided at such a location is because a higher resolution can be obtained if color filters are located closer to transparent electrodes and because color deviation is less when viewed obliquely if they are provided in a shallow depth or on the electrode 14 rather than the electrode 13.
As liquid crystal becomes opaque or transparent according to the principle described for FIG. 1, if a transmission display type is adapted, it can function as a shutter for light. There has recently been developed a liquid crystal printer head which utilizes such function of liquid crystals for the shutter of a copying machine (refer to Nikkei Electronics, 1982, May 10, p.90-p.92). The liquid crystal shutter is constructed in such a manner that a fluorescent lamp is provided as a light source behind a liquid crystal shutter and the light controlled by the liquid crystal shutter is projected to a photosensitive drum via a focusing system. The liquid crystal shutter array comprises 2000 shutters arranged transversely in a zigzag pattern at the pitch of 100 .mu.m and the performance thereof is reported to have the resolution of 10 lines/mm. Such optical printer head is comparable to a semiconductor laser printer in the quality of prints and yet the price is less expensive than a laser printer. For such advantages, the development of an optical printer head is much sought after but the technology still remains at the primitive stage of black-and-white display. The technology has only taken the first step toward the multicolor display.
The present inventor has repeatedly studied and conducted experiments for modifying such printer heads for color printing using the aforementioned color TN methods and came to recognize the critical problem that light tends to leak from the periphery of transparent electrodes to cause black strips (in the case of the photosensitive material of negative working) or white strips (in the case of the photosensitive material of positive working), thereby deteriorating the picture quality to a grave extent.